1. Field of the Invention
The present invention relates to a novel material for electron transportation/injection and emitting layers, and an organic electroluminescence device using the said material, and more particularly to a material for organic electroluminescence (EL) devices providing an electroluminescent device exhibiting a high luminous efficiency, a longer lifetime with reduced driving voltage.
2. Description of Related Art
There has been an increasing interest in developing novel organic materials that cater to organic light emitting device (OLED) applications. Such devices are commercially attractive because they offer the cost-advantageous fabrication of high density pixeled displays exhibiting brilliant luminance with long life times, high efficiency, low driving voltages and wide color range.
A typical OLED comprises at least one organic emissive layer sandwiched between an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton”, which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes through a photoemissive mechanism. To improve the charge transport capabilities and also the luminous efficiency of such devices, additional layers around the emissive layer, such as an electron transporting layer and/or a hole transporting layer, or an electron blocking and/or hole blocking layer(s) have been incorporated. Doping the host material with another material (guest) has been well demonstrated in literature to enhance the device performance and to tune the chromaticity. Several OLED materials and device configurations are described in U.S. Pat. Nos. 4,769,292, 5,844,363, and 5,707,745, which are incorporated herein by reference in their entirety.
The reason for manufacturing an organic EL display with a multi-layered thin film structure includes stabilization of the interfaces between the electrodes and the organic layers. In addition, in organic materials, the mobility of electrons and holes significantly differ, and thus, if appropriate hole transportation and electron transportation layers are used, holes and electrons can be efficiently transferred to the luminescent layer. Also, if the density of the holes and electrons are balanced in the emitting layer, luminous efficiency can be increased. The proper combination of organic layers described above can enhance the device efficiency and lifetime. However, it has been very difficult to find an organic material that satisfies all the requirements for use in practical display applications.
Tris (8-hydroxyquinoline) aluminum (Alq3) is one of the widely used electron transporting material; however, it has an intense green emission and devices using the same exhibits higher driving voltages. Therefore, it is crucial to find an electron transporting molecule that has excellent properties compared to the conventional material in all practical aspects, such as high efficiency, reduced driving voltage and operational stability.
Organic small molecules having imidazole groups, oxazole groups and thiazole groups have been frequently reported as materials for electron injection and transportation layers, as described in the literature Chem. Mater. 2004, No. 16, p. 4556.
U.S. Pat. Nos. 5,645,948 and 5,766,779 disclose a representative material, 1,3,5-tris (1-phenyl-1H-benzimidazol-2-yl) benzene (TPBI), for electron transportation having blue emission. TPBI has three N-phenyl benzimidazole groups, in 1,3,5-substitution sites of benzene and functions both as an electron transporting and an emitting material. However, TPBI has lower operational stability.
U.S. Pat. No. 6,878,469 discloses a compound, wherein the 2-phenyl benzimidazolyl group is linked to the C-2, C-6 positions of anthracene framework. US20080125593and KR20100007143 disclose electron transporting materials comprising imidazopyridyl or benzimidazolyl groups in its molecular skeleton, exhibiting low driving voltage and high efficiency. However, these materials also lack operational stability.